update package.json to have engines node >=0.10.7

npm will display a warning if your engine level is greater than the exact version you currently had the last time you published it.

e.g
npm WARN engine blanket@1.1.5: wanted: {"node":"0.10.7"} (current: {"node":"v0.10.11","npm":"1.2.30"})

https://npmjs.org/doc/json.html